this.searchMode can return null even if a search mode is shown
in the address bar, as the getter uses the current browser as the
context and attempts to use the state data. I try to solve this
by relying on the fact that a tab switch and invalid pageproxystate
results in restoring the search mode state.
Differential Revision: https://phabricator.services.mozilla.com/D228483
935394d9f6